Frank,
I would be very wary of allowing 40 microinches, as this will result in
reduced shelf life due to intermetallic formation. I would estimate that at
least 10 microinches of that 40 microinches and maybe more, is already
intermetallic, leaving you less than 30 microinches as solderable
protection. You can get an idea how thick the intermetallic is by how much
higher than normal the tin percentage readings go. As you get thinner the
tin percentage goes up as the tin in the tin/copper intermetallic becomes a
larger percentage of the total reading.
One thing you must also consider carefully is exactly where are the readings
being taken to test against your requirements, and also what size collimator
is used in testing. "Most Critical Feature" (MCF) is one common way to get
good thickness uniformity. This is especially true if it is taken only at
the crest and with a large collimator. Typically, MCF is taken to be the
finest pitch pads and since due to surface tension these features are much
thicker, you will skew your results. To find minimum thickness, I would
recommend that one set of readings be taken on at least twenty resistor or
capacitor surface mount pads, that the data from this be plotted separately,
and that more than one readings be taken on each pad. This would give a more
accurate take on minimum thickness to make a determination of likely shelf
life. A similar approach can be taken for the finest pitch pads to give an
idea of uniformity and also maximum thickness.
Thanks,
Robert Furrow
Printed Wiring Board Engineer
Strategic Supply Global Account Manager
Supply Chain Networks
Lucent Technologies
978-960-3224 [log in to unmask]
-----Original Message-----
From: Frank Kimmey [mailto:[log in to unmask]]
Sent: Wednesday, August 22, 2001 10:13 AM
To: [log in to unmask]
Subject: [TN] HASL Thickness
Good Day to all,
Hope you can assist with a HASL question (Werner are you out there?)
We are considering some new vendors and they are requesting we reduce our
HASL thickness requirements.
We presently spec 100 to 500 microinches and vendors say they would like to
see 40 to 1000 microinches.
Now the questions:
1. Will 40 microinches cause intermetallic issues?
2. Will 40 microinches cause major shelf live issues?
Expected shelf life 3mos, 6mos, 1 year for sealed in plastic, low
humidity?
3. Since I am not comfortable with 1 mil of tin/lead should 500 microinches
be enough?
I appreciate you comments (Steve, Dave, everyone).
Thanks in Advance,
FNK
Frank N Kimmey, C.I.D.
Senior PCB Designer
Powerwave Technologies
PH. 916-941-3159
Fax 916-941-3195
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